Enzymatic synthesis of poly(ε-caprolactone) in liquified petroleum gas and carbon dioxide

Polycaprolactone (PCL) is a biodegradable, bioresorbable and biocompatible polymer widely used in biomedical applications. Unlike the classical polymerization methods that include the use of a metallic catalyst in organic solvent medium, the main goal of this work is to report the synthesis of polycaprolactone using a commercial enzyme, Novozym 435, as catalyst, together with liquefied petroleum gas (LPG) and carbon dioxide (CO2) as solvent medium in order to establish a clean-technology process. The enzymatic production of PCL was carried out in pressurized LPG medium trough a set of experiments assessing the influence of pressure (120-280 bar), solvent/monomer ratio (2:1-1:2 mass ratio) and enzyme content (by monomer weight, 5-15 wt%) on the reaction yield, number-average molecular weight (M-n), weight-average molecular weight (Mn) and polidispersity index (PI). Kinetic experiments were also conducted in batch using LPG as solvent to evaluate the influence of catalyst content and pressure on reaction yield, M-n, M-w, PI with the reaction time. Furthermore, the enzyme reuse was also evaluated in order to reduce the impact of enzyme cost on the process. Continuous mode polymerizations using CO2 as solvent were also carried out for reactions at 120 and 200 bar, 65 degrees C and solvent/monomer mass ratios of 2:1 and 1:2. Results from ANOVA statistical analysis for the first set of experiments show that the pressure has no significant influence over the parameters evaluated, while the solvent/monomer mass ratio and enzyme content presented significant effect on reaction yield. Polymerization results for the kinetic assays indicated reaction yields up to 81 wt% with Mn up to 15,000 Da and M-w up to 23,000 Da for 20 h of reaction, and PI ranging from 1.2 to 1.7 for the batch reactions and reaction yields up to 60.1 wt% with M-n 21,700 Da and M-w up to 36,800 Da for 10.7 min of reaction, and PI ranging from 1.7 to 2.1 for the PER (Packed Bed Reactor) reactions. The condition of 25 bar, 2:1 solvent/monomer mass ratio, 3 wt% of enzyme, 65 degrees C and 8 h of reaction was selected for the enzyme batch reuse experiments, which indicate technical feasibility of enzyme reuse. Finally, future directions and some perspectives on the field of enzyme-catalyzed biopolymer production in compressed or supercritical fluid media are addressed. (C) 2014 Elsevier B.V. All rights reserved.